Why there is a difference in amount of memory usage between 'onstat -g seg' and 'onstat -g mem' outputs?

 

Problem(Abstract)

You may noticed a difference in amount of memory usage between 'onstat -g seg' and 'onstat -g mem' command outputs. For example; the virtual portion of memory in 'onstat -g seg' is showing about 4GB of memory used, but total of all the memory pools shown by 'onstat -g mem' is only about 1GB in virtual pools.

Cause

You cannot directly compare 'onstat -g seg' with 'onstat -g mem' for virtual memory usage. The 'onstat -g mem' output provides only memory pool information where as the virtual portion of shared memory for the database server includes the following components:

• Large buffers, which are used for large read and write I/O operations
• Sort-space pools
• Active thread-control blocks, stacks, and heaps
• User-session data
• Caches for SQL statements, data-dictionary information, and user-defined routines
• A global pool for network-interface message buffers and other information

 

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